I would keep titles to a minimum as those should be earned the hard way.
but if someone invited 100 people - hell I'd give em a title and a castle
(okay maybe not the castle but some good stuff)
as for general invites I think the players should have a choice - like 1 decor item of their choice OR 1 weapon/armor OR some gold.
This way people everyone has an incentive to invite (some people don't give a fuck about housing decor but would love a cool looking sword) etc.